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m 6 o o ELONGATION AT RUPTURE l l l l l .025 .050 .075 J00 J25 J50 NITROGEN United States Patent 3,201,233 CRACK RESISTANT STAINLESS STEEL ALLUYS Frederick C. Hull, Pittsburgh, Pa., assignor to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed June 13, 1962, Ser. No. 202,220 11 Claims. (Cl. 75--128) This application is a continuation-in-part of application Serial No. 7,449, filed February 8, 1960, now abandoned.

This invention is directed to a family of novel highmanganese stainless steel alloys containing molybdenum and to articles made from these alloys; the alloys exhibiting greatly improved resistance to various types of hotcracking in the as-cast and as-welded condition as well as other desirable properties.

Stainless steel alloys are commonly employed in the manufacture of articles and structures for use at elevated temperature or in corrosive environments. Thus, retorts, pumps, valves, and pressure vessels of various types are often cast in stainless steel. Wrought stainless steel aiticles such as steam chests, piping for boilers and turbines, and other conduits are also commonly subjected to operation in these diflicult environments. In many of these devices welds must be made to unite casting to casting, wrought member to wrought member, and casting to Wrought member.

It has been found that the solidification of stainless steels, and particularly of fully austenitic stainless steels,

from the molten condition is often accompanied by the formation or cracks which tend to impair or destroy the usefulness of the article manufactured. When this phenomenon occurs in castings it is termed hot-tearing, and when it occurs in weld metal it is termed hot-cracking or microfissuring.

The hot-tearing phenomenon often occurs when an austenitic stainless steel casting of complex shape is poured into a mold which is too rigid to allow free contraction of the casting during cooling. Castings having long thin sections immediately adjacent to heavier sections are especially prone to cracking.

The problem of cracking of Welds is particularly acute in the field of high temperature, high-stress applications where Welds must be made to join heavy sections of stainless steel held under restraint. Steam turbines, boilers, and chemical processing equipment require welded sections of this description. The microfissuring of welds actually has two aspects. The first, which is the hot-cracking in the weld deposit which often occurs upon solidifica-tion of molten austenitic stainless steel, has been briefly mentioned above. The second aspect is a cracking phenomenon related to those described above, but which occurs in the heat-affected Zone of either Wrought or cast base metal during welding of heavy sections. The volume of base metal adjacent to the molten weld metal undergoes rapid heating to a temperature close to the melting point followed by cooling to ambient temperature. This rapid heating and cooling cycle, combined with the stresses imposed by hindered thermal contraction, has been found to severely damage the base metal of many stainless steels of the prior art. The damage is evidenced by cracking which originates in the grain boundries of the base metal.

Stainless steels of the AISI types 304, 316, 321, and 347 were developed many years ago to resist general corrosion, pitting, and grain boundary attack. When it became evident that these austenitic stainless steels, and in particular AISI types 316, 321, and 347 had much higher creep strength than ferritic steels, they were applied in many high temperature, high-stress situations as the best available materials. The application of fully austenitic alloys such as the alloys of the AISI 300 series mentioned above, is limited by the susceptibility of certain of these alloys to hot-tearing when cast to shape as large components and to hot-cracking during the welding of restrained joints.

Since the apparatus used at elevated temperatures, such as steam chest, stearn-turbine piping, and chemical retorts, has great importance for industry, the problems of hottearing and Weld cracking have been the subject of intense investigation. These cracking phenomena appear to result from the lack of ability of the metal grains to hold together when subjected to tensile stresses at temperatures just below the solidus or perhaps in some cases While trace films of liquid are still present between some grains. The exact mechanism by which this high temperature cracking occurs is not understood, nor is there any known method for healing the cracks once they have formed. Wide variation has been found in the susceptibility to cracking even within a given grade of steel, and there is as yet, no satisfactory explanation for this variation. Uncertainty as to the quality of the welded joints has meant that excessive time and money are expended for testing, inspection, and repair welding to prevent service failures in critical applications such as have been mentioned above.

It is a primary object of this invention to provide novel stainless steel alloys having improved resistance to hottearing and hot-cracking and high creep rupture strength, and which are suitable for use as castings, as wrought products, and as Welding rod, which include, in predetermined proportions, the elements iron, chromium, nickel, and manganese, with predetermined amounts of molybdenum.

It is a further object of this invention to provide a novel stainless steel alloy for use as castings, as Wrought members, and as welding rod having improved resistance to hot-tearing and hot-cracking, which includes, in predetermined proportions, the elements iron, chromium, nickel, manganese and molybdenum, in which the total percent of manganese and molybdenum is a critical consideration.

A further object of this invention-is to provide a substantially austenitic stainless steel alloy having good resistance to hot-cracking and hot-tearing, and improved strength and corrosion resistance, which includes in predetermined amounts the elements iron, chromium, nickel, manganese, molybdenum and at least one carbide stabilizer selected from the group consisting of titanium and columbium.

It is still another object of this invention to provide a welding electrode capable of depositing a stainless steel weld bead resistant to microfissuring as a joint between substantially austenitic stainless steel members and having an essentially austenitic structure which includes, in predetermined amounts, the elements iron, chromium, nickel, manganese, and molybdenum.

Yet another object of this invention is to provide a crack resistant substantially austenitic stainless steel having relatively high strength including, in predetermined proportions, the elements iron, chromium, nickel, manganese and molybdenum as a base alloy, with small but critical additions of vanadium, boron, zirconium and nitrogen.

Still another object of this invention is the provision of weldable austenitic stainless steels which are useful at sub-Zero temperatures and specifically for cryogenic applications involving the handling of liquefied gases, such as liquid nitrogen, hydrogen or helium.

Other objects of the invention Will, in part, be obvious, and will, in part, appear hereinafter.

The nature of the invention will be more readily understood from the following description taken in conjunction with the figures, in Which:

FIGURE 1 is a typical CPT test (which is hereinafter defined) curve in which cracking index is plotted against arbitrary mold numbers;

FIG. 2 is a graph plotting CPT test mold numbers for a cracking index of 40 against Weight'percent of some common alloying additives in stainless steel;

FIG. 3 is a graph similar to FIG. 2 for certain other additives in stainless steel;

FIG. 4 is a hot-cracking curve for manganese in which CPT test mold numbers for cracking indices (CI) of 40 and zero are plotted against weight percent of manganese;

FIG. 5 is a graph in which percent molybdenum is plotted against percent manganese and contours are drawn to indicate compositions having equal cracking characteristics;

FIG. 6 is an illustration of a simplified impact test;

FIG. 7v is a graph in which bend angle is plotted against chromium content for alloys of this invention;

FIG. 8 is a graph in which bend angle is plotted against the composition function;

FIG. 9 is a graph showing the effects of nickel and manganese and delta ferrite on the cracking of stainless steels in which average cracking index in a No. 6 CPT test mold is plotted against percent nickel;

FIG. 10 is a graph in which stress rupture strength is plotted against nitrogen content for weldable stainless steels and for high strength weldable stainless steel;

FIG. 11 is a graph similar to FIG. 10 in which creep strength is plotted against nitrogen content for the same groups of steels;

FIG. 12 is a graph in which elongation at rupture is plotted against nitrogen content for alloys of this invention; and

FIG. 13 is a graph in which yield and tensile strengths of alloys at room temperature and 1025" F. are plotted against nitrogen content.

This invention is directed to a family of weldable stainless steel alloys containing iron, nickel, chromium and molybdenum which owe their distinctiveness in large degree to the uncommonly high proportion of manganese which they contain and the unique relationship between manganese and molybdenum. This alloy family includes member alloys having a wide range of characteristics which result from modifications of the basic composition either by changing the proportions thereof and/ or adding other alloying elements. These alloys are fully or highly austenitic. I

In the broadest sense the fully austenitic weldable alloys of this invention comprise, by weight, from 14% to chromium, from 15 to nickel, from 7.5% to 15% manganese, from 0.5% to 3.75% molybdenum, the total of manganese and molybdenum lying in the range from 9% to 16%, from 0.01% to 0.08% carbon, from 0.01% to 0.35% nitrogen, up to about 1% silicon, up to 1% vanadium, up to .03% boron, up to .06% Zirconium, and the balance iron with, in some cases, small amounts of additives and small amounts of incidental impurities, the F(c) value of the composition being greater than 14 and the (c) value lying in the range from 1 to 3.75. The composition functions F(c) and (c) will be defined hereinafter.

A preferred range of composition for the fully austenitic weldable alloys of this invention is, by Weight, from 14% to 18% chromium, from 15% to 26% nickel, from 9% to 14% manganese, from 1% to 3% molybdenum, the total of manganese and molybdenum lying in the range from 12% to 15%, from 0.01 to 0.06% carbon, from 0.01% to 0.25 nitrogen, up to about 0.6% silicon, up to 1% vanadium, up to 0.02% boron, up to 0.04% zirconium, and the balance iron and small amounts of incidental impurities, the F(c) value of the composition being greater than 14 and the (c) value lying in the range from 1 to 3.75.

An austenitic, highly weldable alloy Within the broad range defined above, which is suitable for many conventional stainless steel applications as Well as for cryogenic applications, comprises, by weight, from 14% to 18% chromium, from 18.5% to 22% nickel, from 8% to 13% manganese, from 1.0% to 3% molybdenum, the total of manganese and molybdenum lying in the range from 11% to 16%, from 0.01% to 0.06% carbon, from 0.01% to 0.09% nitrogen, from 0.08% to 1% silicon, and the balance iron except for small amounts of incidental impurities, the F(c) value of the composition being greater than 14 and the (c) value lying in the range from 1 to 3.75. An alloy similar to that just described, butcontaining from 0.10% to 0.25% nitrogen is especially useful as a high strength, weldable, austenitic stainless steel at cryogenic temperatures.

A high strength austenitic weldable stainless steel alloy of this invention comprises, by weight, from 14% to 18% chromium, from 18.5 %'to 22% nickel, from 8% to 13% manganese, from 1.0% to 3% molybdenum, the total of manganese and molybdenum lying in the range from 11% to 16%, from 0.15% to 0.5% vanadium, from 0.002% to 0.03% boron, from 0.002% to 0.06% zirconium, from 0.1% to 0.35% nitrogen, from 0.01% to 0.06% carbon, up to about 0.6% silicon and the balance iron and small amounts of incidental impurities, the F(c) value of the composition being greater than 14 and the (c) value lying in the range from 1 to 3.75.

A principally austenitic, highly weldable stainless steel alloy of this invention, having from 2% to 10% delta ferrite therein to increase resistance to hot-cracking and hottearing and which is especially suitable for castings and for welding electrodes, comprises, by weight, from 14% to 18% chromium, from 5% to 15 nickel, from 8% to 13% manganese, from 1% to 3% molybdenum, the total of manganese and molybdenum lying in the range from 11% to 16%, from 0.01% to 0.08% carbon, from 0.01%

to 0.09% nitrogen, from 0.08% to 1% silicon and the bal-' ance iron except for small amounts of incidental impurities, the F(c) value of the composition being from 7.5 to 11.5 and the (c) value lying in the range from 1 to 3.75.

Stabilizing additions of from 0.25% to 1.5% total of columbium and/or titanium may be added to the alloys of this invention.

Larger amounts of titanium may be employed to develop precipitation hardening. Thus, a further alloy of this invention is a precipitation hardenable, austenitic, weldable stainless steel alloy, comprising, by Weight,

from 14% to 18% chromium, from 15% to 30% nickel,

In general the alloys of this invention are characterized by good resistance to hot-cracking and hot-tearing as indicated by a CPT test number (hereinafter defined) of less than 11.

For the purpose of strengthening these alloys, tungsten and vanadium may be used, in combination with 0.5% to 3.75% molybdenum, in amounts of up to 6.5% tungsten and up to 1% vanadium, such that the equivalent molyb denum content (defined as actual molybdenum plus onehalf the tungsten plus twicethe vanadium) lies in the range 0.5% to 3.75%. Molybdenum should be present in the minimum amount of at least 0.5% in combination With manganese to provide substantial resistance to hotcracking.

In the alloys of this invention, the hot-cracking and hot-tearing resistance approach optimal values when the total of manganese and molybdenum is in the range from 12% to 15%. However, substantial benefits are observed in the range from 11% to 16%, and a manganese molybdenum total of as low as 9% may be tolerated where crack promoters are present only in small quantities.

The impurities phosphorus and sulfur are maintained within limits normal for comparable austenitic stainless steels; that is, 0.04% maximum phosphorus and 0.03% maximum sulfur.

In accordance with this invention a number of considerations enter into the selection of the optimum compositions of these stainless steels depending upon the intended service conditions; for example, at high temperatures, in contact with corrosive liquids or in cryogenic equipment. The properties of importance include strength, oxidation and corrosion resistance, ductility, impact strength, stability in long-time high temperature service, dimensional stability and magnetic permeability at low temperatures. The composition may also be related to the form in which the alloy is used; i.e., wrought, cast, or as welding electrodes, and to the thickness and rigidity of the elements being joined. This invention is directed primarily to a new family of stainless steels which have exceptional resistance to hot-cracking and hot-tearing when welded or cast. These and other factors that are necessary to define the composition requirements for various applications of the stainless steel alloys of this invention are discussed in part in the following sections under appropriate headings.

HOT-CRACKING In order to provide a technique for the convenient quantitative evaluation of the hot-tearing and hot-crack ing characteristics of alloys, I have developed the Cast Pin Tear test, hereinafter referred to as the CPT test, which has been fully described in the Welding Journal, volume 38, April 1959, pages 176-S to 181-8. In brief, the CPT test involves casting small samples of an alloy in a series of copper molds graduated in size under conditions which impose a tensile strain on the sample as the mold expands and as the casting contracts during solidification and cooling. The geometry of the particular mold that will produce a certain degree of hot-cracking provides a means of classifying alloys in order of merit or of predicting service performance.

From an examination at a suitable magnification of the surface cracking of the pins after cooling, the cumulative percentage of circumferential cracking (cracking index) is determined and plotted against arbitrary mold numbers as illustrated in FIG. 1 to provide a measure of the susceptibility of the alloy to cracking. The molds .are graduated so as to serially progress in relatively regular steps from long, slender molds to short, heavy molds. The long slender molds .(of the aforementioned series) which have the smaller arbitrarily assigned numbers produce severe cracking, whereas the short, heavywalled molds which have the larger arbitrarily assigned numbers crack the pin less severely, or not at all.

The results of the CPT test have been found to have good correlation with the performance of weld deposits in actual welds. It is not difficult to understand the correlation found between the test results and the actual cracking observed in alloys cooled from the molten state, because the CPT test is based on the resistance to cracking of a cast structure as it solidifies. Although the ability to predict cracking behavior in the heataffected zone of a wrought aloy during welding from the same test results is not nearly as obvious, the CPT tests have been found to correspond to actual welding experience and to correlate with certain results of the RPI test, as will be explained more fully below.

One technique currently used to predict the weldability of wrought stainless steel is Rensselaer Polytechnic Institute (RPI) hot ductility test, described in Further Studies on the Hot Ductility of High Temperature Alloys, Welding Research Council Bulletin Series, Number 33, February 1957. In this test, A inch diameter specimens of an alloy are heated, by passing a large electric current through them, through a time-temperature cycle that simulates conditions in the heat-affected zone of a weld. When the specimens are pulled at different temperatures of this cycle, it is found that heating close to the melting point has damaged the material, as evidenced by a decrease in its ductility on cooling compared to the ductility measured at equivalent temperatures on heating.

A correlation has been established between the cracking susceptibility of stainless steels as measured by the CPT test and the performance in an RPI type hot ductility test, when the maximum temperature on heating is that at which the strength drops to zero. Without going into the details of a procedure not a part of this invention, it should be understood that it is possible to predict the behavior of stainless steels in the heat-affected zone of the base metal from CPT test results. Although other factors, such as prior thermal and mechanical history of the base metal may influence heat-affected zone cracking, it appears that the primary factor is the alloy composition. Summing up, the CPT test may be used to evaluate austenitic stainless steels for crack resistance in castings, weld metal, and base metal adjacent to welds.

By means of the CPT test, I have evaluated the individual effects of many different alloying elements on the cracking of chromium-nickel stainless steels. The general nature of the test results may be seen in FIG. 1 where the data for a ternary iron-chromium-nickel alloy are graphically presented. The cracking index, or cumulative percentage of circumferential cracking in any one specimen for this alloy ranges from 0 to nearly 180. A cracking index of 40 has been found to be convenient for the purpose of comparison of alloys. It will be noted that the ternary alloy tested in FIG. 1 has a cracking index of 40 at mold number 11. With this in mind, it is interesting to observe FIG. 2 in which the effects of various alloying additions on the ternary alloy of FIG. 1 are set forth in terms of mold number at a cracking index of 40, as a measure of the resistance to hot-tearing and hot-cracking. The smaller the mold number, the more resistant the alloy is to hot-cracking. A high nickel content of 20% was purposely selected for the base composition in this study so that even with substantial additions of ferrite formers such as chromium, molybdenum, vanadium and tungsten, the alloy would remain fully austenitic as cast and the effects of the individual alloying elements would not be obscured by variable amounts of delta ferrite in the structure.

FIG. 2 clearly demonstrates that each element exerts an individual effect on the hot-cracking characteristics of the basic alloy which, in many cases, is substantially different from that of the other elements tested. For example, manganese, molybdenum, tungsten and chromium materially increase the resistance to cracking, each to a different degree. On the other hand, copper, silicon, columbium and titanium have a pronounced deleterious effect, each to a different extent. Copper in amounts of less than 0.25% may be found in these alloys since small amounts are often present in scrap. Cobalt and vanadium are almost identical in their results, as far as cracking is concerned, and do not appear to be either beneficial or deleterious. Hence, cobalt may be substituted for equal amounts of nickel up to at least 5% with some bene ficial effect on the characteristics of my alloys. Nickel shows only a slight detrimental effect.

Similar information to that of FIG. 2 is presented with respect to certain other elements in FIG. 3. It should be noted that the horizontal scale indicating the amounts of the elements covers only up to 0.25% whereas in FIG. 2 the scale for alloying elements went up to 5%. It is quite apparent that even as little as 0.01% of boron is highly detrimental. Also zirconium and hafnium, in amounts of 0.1%, greatly impair the cracking resistance of the alloy. Nitrogen, in contrast, reduces hot-cracking. From other tests made, the beneficial or detrimental effects of the additives shown in FIGS. 2 and 3 for the 16% chromium, 20% nickel, balance iron alloys, have been determined to be similar for stainless alloys having other proportions of nickel, chromium, manganese, molybdenum and iron.

As a guide in correlating CPT test results with practice, it has been found that most casting and welding applications are satisfied by an alloy having the crack resistance indicated by a CPT mold number of '8 to 10, for a cracking index of 40. Alloys having CPT mold numbers of or 11 are very useful alloys having many applications. Alloys having a CPT mold number higher than 12 or 13 are only marginally applicable, for cracking is common, and serious difficulties may arise in casting and Welding. I many prior art fully austenitic alloy compositions are found. 2

Because of the improvement in resistance to hot-cracking I found for manganese additions up to 5% (FIG. 2), I decided to thoroughly investigate the effect of manganese on hot-cracking of iron-chromium-nickel alloys. The results of this study are summarized graphically in FIG. 4. For this work, the chromium content was held constant at approximately 16% while the nickel content ranged between 14% and The amount of nickel in the alloy assured a fully austenitic structure without dependence on the manganese content. In the graph, the CPT test mold numbers for cracking indices ot-40 and zero are plotted against the manganese content, which is varied over the wide range shown. It will be noted that the beneficial efiect of manganese on the alloy reaches a maximum at about 14%, and outstanding crack resistance is obtained in the range of 11% to 16% manganese, but that the slopes of the curves are such that substantial beneficial eflect extends to the limits of the range 7.5% to 18% manganese. This range of manganese content includes quaternary compositions having higher resistance to cracking than any other quaternary fully austenitic stainless steel containing iron, chromium, and nickel. In the alloys of this invention, the manganese, which has very little eltect on delta ferrite formation as will be shown later, is not relied upon to provide the austenitic or face-centered-cubic crystal structure, since that is the function of the nickel addition; the manganese is present primarily to reduce the suscep tibility to cracking.

Finally, in the course of investigating the effects of various alloying additions for the purpose of increasing the strength of Fe-Cr-Ni-Mn alloys, it was discovered that not only did molybdenum increase the strength considerably, as might have been anticipated from the prior art, but it furthermore actually reduced susceptibility to hot-cracking. In FIG. 5 CPT test data are plotted from a large number of fully austenitic alloy samples containin approximately 16% chromium and from 14% to 20% nickel in which the manganese and molybdenum contents have been varied over a Wide range. As the data are presented in FIG. 5, contours of equal amounts of cracking have been drawn. Compositions having a mold number of about 8.5 or smaller in FIG. 5 are characterized by a resistance to cracking suficientto permit casting of shapes and welding of restrained joints of heavy sections of nearly all usual types or forms without microfissur ing in welds or castings, or base-metal cracking in the heat-affected zone of welds.

It is in this range above 12 or 13 that With respect to these austenitic iron-chromium-nickelmanganese-molybdenum alloys, it will be observed in FIG. 5 that the greatest resistance to hot-cracking occurs at about l.5% molybdenum and 12% manganese. Excellent resistance to hot-cracking is exhibited by alloys containing, as preferred ranges, from 1% to 3% molybdenum, and from 9% to 1% manganese. If an alloy having greater strength is desired, improvedstrength can be obtained by further increasing the molybdenum content, but the increase of molybdenum produces slightly increased susceptibility to hot-cracking. In order to minimize the hot-cracking characteristic in alloys particularly suitable for welding, and having molybdenum in amounts greater than 1.5%, it is preferred to decrease the manganese content by 1% for each 1% increase in molybdenum. Thus, for example, at 3.75% molybdenum it ispreterred to use 9.75% manganese. Stated more generally, the minimum susceptibility to hot-cracking at a given molybdenum content is obtained when the total of manganese and molybdenum amounts to from about 12% to 15% by weight, of the alloy. .These five component compositions have an exceptional combination of strength and resistance to microfissuring or cracking. Also, equally good alloys are produced by wholly or partially substituting tungsten for the molybdenum, using two parts by weight of tungsten for each part of molybdenum it replaces. For example, in Table VI, in which numerous examples of the alloys of this invention appear, several alloys are listed in which tungsten is substituted for part or all of the molybdenum. These alloys have equally good resistance to hot-cracking because, as shown in FIG. 2, tungsten as well as molybdenum reduces the hot-cracking susceptibilityof stainless steels. However, since the tensile and creep properties of the steels of this invention, containing all or part tungsten are not significantly better than those containing only molybdenum, and since tungsten is more expensive than molybdenum per pound and twice as much is needed to produce an equivalent effect, I prefer to use molybdenum alone in my steels for economic reasons. Vanadium does not appreciably afiectthe hot-cracking characteristics of my steels.

STRENGTH It has been a common practice in the prior art to add various amounts and proportions of molybdenum, tungstem and vanadium to stainless steels to increase the strength; At the lower composition levels these elements act mainly as solid solution strengtheners; but, at higher levels, precipitates form during heat treatment or duringelevated temperature testing. I have found in these types of stainless steel alloys'that creep and rupture strength increase in an approximately linear manner from 0 up to about 8% equivalent molybdenum, defined as percent, Mo+ +2 (percent V) OXIDATION RESISTANCE During high temperature solution heat treatment of these alloys or several thousand hours of aging at 1500" F. and 1350 F., I have observed that, at the preferred chromium content of 16%, the molybdenum should be kept below 4% and vanadium below 0.5% (when it is present in combination with some molybdenum) in order to avoid severe oxidation which can assume the propor- STRESS RUPTURE DUCTILITY Insofar as rupture ductility is concerned in wrought materials, this increases along with strength to a maximum at about the level of 4% molybdenum (or its equivalent) and thereafter decreases. The increased ductility is believed to be due to an optimum amount and distribution of precipitate which forms as beads and plates along the grain boundaries to block boundary sliding. Too little precipitate allows the grain boundaries to slip and fractures occur at junctures between three grains. On the other hand, a continuous film of the brittle sigma phase is detrimental and reduces ductility. In cast structures with a lower amount of grain boundary area per unit volume, the molybdenum content should be below 4%. Actually, the whole composition balance of the alloy is related to the ductility and impact strength.

IMPACT STRENGTH One of the primary distinctions between my invention and the prior art is the discovery in the field of austenitic stainless steels of the need for control of nickel at a higher level than has been recognized as necessary. The literature contains many references illustrating how fully austenitic stainless steels can be produced by substituting one or more of the elements manganese, carbon and nitrogen for all or a major part of the nickel. The aim of many of these developments has been to find a cheaper steel that contains no delta ferrite as cast and whose martensite formation temperature has been lowered to approximately room temperature or below. Manganese, carbon and nitrogen do act as substitutes for nickel in these respects. On the other hand, there is a major difference between these elements and nickel with respect to the high temperature stability of the alloys. Carbon and nitrogen do not appreciably affect the sigma forming characteristics of an alloy, whereas manganese actually promotes sigma formation, in contrast to nickel which tends to suppress it, as well now be shown. Thus, many of the prior art alloys containing high manganese, high carbon and/or nitrogen and low nickel content experience rapid deterioration of impact strength upon aging in elevated temperature service.

In connection with the effect of various alloying additives upon the impact strength of the alloys of this invention, a great number of alloys were investigated and tested after accelerated aging by a simplified impact test, the results of which could be correlated with conventional Charpy test results. In these tests, aging was conducted at 1500 F. for times up to 1000 hours. This condition was taken to simulate the effects of the much longer times encountered at service temperatures which are usually in the range'of 1000 to 1200 F. Alloy samples were prepared by levitation melting in an argon atmosphere and 20 gram tapered specimens about A inch in diameter were cast in copper molds. After aging times of zero, 17, 100 and 1000 hours at 1500 F., the cast specimens were notched half-way through with a hacksaw, held in a vise, and subjected to a simplified impact test. Using the arrangement shown in FIG. 6, from 1 to 20 blows of a one and one-half pound hammer were required to break the notched pins or bend them to a 90 angle. The bend angle at fracture was the value most easily correlated with conventional Charpy test results. FIG. 7 shows the typical curves obtained'by this technique for an alloy containing 20% nickel, 10% man- 10 ganese, 3% molybdenum, 0.1% silicon, chromium in various amounts from 12% to 22% and the balance iron. An alloy containing 16% chromium, 20% nickel, 10% manganese, 2% molybdenum, 0.1% silicon, the balance iron, was used as a base to determine the effects of additions of tungsten, vanadium, columbium, titanium and silicon. In a similar way, with slightly different bases, the effects of chromium, nickel, cobalt, manganese and molybdenum additions were evaluated. These results can be summarized in the form of an equation which reduces the composition of an alloy within the scope of this invention to a single variable (c) which is uniquelyrelated to the impact properties after aging (alloy components in weight percent).

The larger the value of (c) the more precipitate was found in the structure after aging. This precipitate, which in large amounts seriously embrittles the alloy, is believed to be the sigma phase. If the 1000 hour notched impact bend angles at fracture for all these alloys are plotted in one graph against (c), the results lie within a relatively narrow band as shown in FIG. 8. Carbon and nitrogen up to 0.2% do not appreciably change this relationship. It has been found that in order to have good stress-rupture ductility, (c) should be greater than about 1.0 and less than 4.0 and to avoid severe loss of impact strength on aging in high temperature service, (c) should be less than 3.75 and preferably less than 3.0.

STABILIZING ADDITIONS Since the problem of preventing intergranular corrosion of stainless steels in some applications is a serious one, carbide stabilizers such as titanium or columbium are frequently added, in minimum amounts of about 8 and 10 times the carbon, respectively, to stainless steels to prevent intergranular precipitation of chromium carbides with subsequent accelerated corrosion.

Columbium is also effective in increasing strength at high temperature so that smaller ratios of this element may be added where corrosion resistance is not a primary consideration. Small additions of titanium tend to increase hot ductility (see Table XXIV and the text associated therewith) and may be made even in cases where corrosion resistance is not of primary importance. Much larger quantities of titanium in the range of 1.5% to 2.5%, by weight, may be added to obtain precipitation hardening in the alloys of this invention, and this will be discussed in detail hereafter.

It was previously shown in FIG. 2 that titanium and columbium seriously increase the susceptibility to hotcracking of a chromium-nickel stainless steel. The CPT test was again used to evaluate the effects of these elements in alloys with preferred amounts of manganese and molybdenum and both with and without delta ferrite, as presented in Table I.

Table l EFFECTS OF ALLOYING ELEMENTS A'ND DELTA FER- RITE ON SUSCEPTIBILITY iOF STAINLESS STEELS TO HOT-CRACKING The relationship of the carbide stabilizers to the hotcracking properties is also'found to hold true in the ironchromium-nickel-manganese-molybdenum alloys, Where it is clear that titanium and columbium significantly increase the'hot-cracking tendencies. It is important to note, however, that because of the generally low susceptibility to hot-cracking that has been discovered in the iron-chromium-nickel-manganese-molybdenum alloys with or without tungsten and vanadium, it is possible to employ either titanium or columbium as a stabilizing element in substantial quantities from 0.25% to 1% or slightly higher and still produce alloys which have lower susceptibility to hot-cracking than stainless steels currently in use, and which are sufficiently resistant to cracking to be welded in heavy sections and to be cast into complex shapes. As shown in Table I, even with 1% titanium or 1% columbium, the CPT test mold numbers are 9.5 to.10.4 for such alloys, actually less than the mold'number of 11 for the 16% chromium-20% nickel base composition. Without manganese in the alloys, the mold numbers reach the highly undesirable values of 13.5 and 13 when 0.75% titanium and 1% columbium are present.

The discussion of this new class of stainless steels would be incomplete without some reference to the effects of delta ferrite and methods of controlling ferrite within specified limits or avoiding it in alloys where a fully austenitic structure is desired. In the prior art the conventional means for decreasing the hot-cracking of austenitic stainless steels, on casting complex shapes in a foundry or on welding restrained joints, has been the preparation of steel castings or weld deposits whose cast structures contain a controlled percentage of delta ferrite. In the alloys of this invention, likewise, improved resistance to cracking can be. obtained when from 2% to 10% of the total volume of the cast alloy is delta ferrite. However, since delta ferrite is known to accelerate the formation of the brittle sigma phase in alloys during elevated temperature service, it was a primary object of this invention to disclose alloys so resistant to hot'-crack' ing that sound castings and weld deposits could be made that were fully austenitic.

The methods for obtaining a fully austenitic structure or for controlling the amount of delta ferrite in cast alloys and weld deposits of stainless steels are well under-.

stood in the prior art. In general, the amounts of chromium and nickel in the alloy are the variables most usually controlled, but other additives in the alloy will also exert aneffect upon the phase relationships which must be taken into consideration. For example, if strength and/or corrosion characteristics dictate the levels of solid solution hardeners (molybdenum, tungsten, and vanadium) and stabilizers (titanium and columbium) desired, and melting practice produces a certain level of carbon and nitrogen, if deoxidation yields about 0.5% silicon, there are only chromium, nickel, and manganese left to affect the structure. Since chromium must exceed about 14% to give the steels their stainless characteristics, and manganese has little effect on the amount of delta ferrite (as will be apparent from the following), the primary control over the structure resides in the selection of the nickel content.

Five factors enter into the selection of the optimum nickel content for fully austenitic alloys of this invention;

(1) Because nickelincreases the hot-cracking suscep-y tibility by about. 0.5 CPT test mold number for ever 5% excess nickel, nickel should be kept low.

(2) In wrought products it is desired to keep delta ferrite out of the cast ingot or wrought material in order 7 ferrite-containing electrode, if the nickel content of the base metal is too high, dilution of the Weld head by the base metal will decrease or eliminate the delta ferrite from the Weld and may cause hot-cracking. From this standpoint the base metal should be as low in nickel as items (2) and (4) will permit.

V (4) In high temperature service, nickel counteracts the sigma forming tendencies of Cr, Mn, Mo, W, V, Si, Cb and Ti, so the nickel content should be high enough to prevent serious embrittlement.

(5) Finally, from practical considerations, higher nickel than needed for items (2) and (4) is uneconomical and wasteful. i e

For the composition ranges covered by this invention I have determined the nickel equivalents of the various additives so that a fully austenitic alloy or one having a predetermined delta ferrite content can be obtained as desired. The high level of manganesein the alloys of the present invention might be expected to introduce a variable not met with in the prior art. The effects of nickel on the amount of delta ferrite in alloys containing 16% chromium'and 11% or 14% manganese plus additions of titanium, columbium, vanadium, tungsten, silicon, nitrogen and molybdenum are shown in Table II.

Table 11 EFFECTS or VARIOUS ELEMENTS ON MroaosTRUoTURE or oilsr STAINLESS STEELS Percent Nickel for'the Indicated Microstruetures 'Principsl Alloying Elements, Weight, Percent, Balance Ni and Fe 10% 2.5% Fully Delta Delta Austen- Ferrite Ferrite itic 16 Cr, 14 Mn, 1.75 '1 9 V 14 15 16 Cr, 14 M11, 1.02 C 7 11 11. 5 16 Cr, 14 M11, 2.98 V 14. 5 16. 7 17 I 16 Or, 14 Mn, 1.54 W 10 13. 3 14 16 Cr, 14 Mn 4. 5 10.7 11. 5 16 Or, 14 Mn, 2.89 Si 7. 5 12. 0 13 16 Or, 14 Mn, 0.11 N 3. 5 .6. 7 8 16 or, 11 Mn, 2 Mo 7 13. 3 14 From these and similar determinations ithas been found that the effects of the. alloying elements to he as follows in producing delta ferrite in my steels, if nickel is assigned an austenite forming tendency of +1 for each Weight percent nickel (a minus sign indicates a tendency to form delta ferrite) Table III AU STENITE(+) AND DELTA FERRITE() FORMING "Using the above values, the following equation has been derived which enables one to predict the microstructures of castings and weld deposits of alloys of this invention. When the composition function, F(c) in the following equation, exceeds about 13 to 14 the alloy will be substantially austenitic, whereas when it lies'between 7.5 and 11.5 the alloy will contain about 10% to 2% F(c)=(percent Ni)+0.02 (percent Mn)+37 (percent C-l-percent N)2.0 (percent V)1.8 (.percent Ti) 0.8 (percent Mo)0.7 (percent Cr16)0.5 (percent W)0.4 (percent Si) -0.1 (percent Cb) Of particular interest in the application of the present invention is the unexpected discovery that manganese has practically no effect on the austenite-delta ferrite limits in cast alloys (nickel is 50 times more effective than manganese). However, manganese helps suppress the transformation of austenite to martensite if the total alloy content of chromium and nickel is low, as for example, at 16% chromium and 10% nickel.

Another item of interest is the very small effect of columbium per se on the amount of delta ferrite. There is, however, an interaction between (columbium and/ or titanium) and (carbon and/ or nitrogen) such that if both are present in substantial amounts carbon and nitrogen appear to be removed from solution as carbides and nitrides and delta ferrite appears in the microstructure at a somewhat higher nickel content than predicted by the equation. r

The effect of the nickel content and the delta ferrite content on the hot-tearing of stainless steels is illustrated clearly in FIG. 9. In that figure, cracking index in the number 6 CPT test mold has been plotted against nickel content for constant chromium of 16% and manganese contents of and 14%. In the fully austenitic range, cracking for both alloys decreases gradually as nickel decreases from 20% to 11%. However, as delta ferrite appears in the as-cast pins, cracking decreases at first abruptly and then more slowly as nickel decreases. It is significant that even with alloys containing delta ferrite, a substantial and unexpected decrease in susceptibility to hot-tearing is produced by the addition of 14% manganese.

The hot-tearing characteristics of a molybdenum-containing alloy were determined for alloys containing 16% chromium, 11% manganese and 2% molybdenum. The nickel content of these alloys was varied from 6% to 20% As in the alloys which did not contain molybdenum, an especially lowsusceptibility to hot-tearing was found below about 11% nickel; this corresponds to a delta ferrite content of about 7% by volume in the as-cast structure.

In Table I, alloys of iron-chromium-nickel-manganese which have been solid-solution hardened with molybdenum and stabilized with either titanium or colurnbiurn, and which contain significant amounts of delta ferrite may be compared with similar alloys which do not contain delta ferrite, as to these hot-tearing properties. It will be observed that improvements in resistance to hot-tearing are effected by the presence of the delta ferrite phase.

STRENGTH-CARBON AND NITROGEN still higher strength alloys are needed, additional alloyingelements and strengthening mechanisms must be relied upon. In the prior art, carbon and nitrogen individually and in combination have been used many times to strengthen stainless steel. However, based on the hotcracking consideration andthe effect of carbide precipitation on impact strength, it is necessary to keep the carbon content low in the steels of this invention, that is, generally below 08% and preferably below 06%. Accordingly, an increase in carbon content is not a feasible method of increasing strength in these alloys. On the other hand, .FIG. 3 shows nitrogen to have the beneficial effect of reducing hot-cracking. Table IV and FIGS. 10 and 11 show that Heat No. 7470 with 0.13% N has significantly higher stress rupture and creep strength than the heats of lower nitrogen content.

Table IV STRESS RUPTURE AND CREEP PROPERTIES 1,200 I .1,000 Hr.

Stress for .01% Heat N 0. Percent per hour mini- Nitrogen Rupture Percent mum creep rate Stress, Elongaat 1,200 F., psi.

p.s.i. tion 7392 016 15, 000 55 14, 000 7431 019 16, 500 32 10, 000 XMM2054 .040 19,000 15, 000' 7454 041 20, 000 76 17, 000 7470 13 23, 000 20 22, 000

Yield and tensile strengths are higher as well.

Nitrogen contentsin excess of about 0.15% are not desirable in the simple Fe-Cr-Ni-Mn-Mo steels, because a lamellar rain boundary precipitate, which has a detrimental effect on ductility, forms during high temperature service. It is clear then, that moderate increases in nitrogen content will increase strength without serious detrimental effects.

LOW TEMPERATURE AUSTENITE STABILITY OF WELDABLE STAINLESS STEEL casting was tested at '423 F. The fractured specimen which had elongated 42% and had 30% reduction of area showed no detectable ferrite as measured at'roorntemperature. "Thisspecific casting had a composition of 16.2% Cr, 16.5%Ni, 11.1% Mn, 0.42% Si, 3.02% Mo, 0.04% C, 0.008% S, 0.024% P and balance iron; Howjever, this test left unanswered the question of whether martensite would have formed if the composition was slightly different, the test temperature lower or the deformation more severe.

In order to determine the effect of these variables, a

series of small chill castings was made with-variable nickel content and with the remaining elements held constant at 16% Cr, 11% Mn, 2% .Mo, 0.004% C, 0.002% N,

0.03% Si, balance iron. A block of hardened steel and a A" diameter by A" high cylinder of the alloy to be tested were cooled in liquid nitrogen. These were then placed on an anvil and the specimen quickly upset by harnmering to Ms high. The equivalent ferrite or martensite as measured at room temperature after this 50% reduction in height of a cylinder at 320 F. was as follows:

An alloy containing 16% Cr, 14.5% Ni, 11% Mn, 2% Mo, balance iron thus has an M temperature of -320 F. The M temperature is the lowest temperature at which a sample may be deformed without producing martensite- The M temperature is normally about 600 F. higher than the M temperature, the temperature at which martensite forms on cooling and thus is a more severe standard of austenite stability. From some similar tests, the effects of changes of Ni, Cr, Mn, Mo, Si, C and N from this base on M would be approximately as follows:

AM percent I For example, ifthe nickel content of the above alloy were increased only 1% to 15.5%, M would be approximately -430 F. With the normal contents of carbon and nitrogen specified in these steels and with nickel greater than 15%, the austenite in the alloys is fully'stable down to liquid helium temperatures.

The specification thus far has been devoted primarily to a description of various quantitative relationships which I have discovered that relate the composition of stainless steels to the major characteristics governing their use. 4

These characteristics includehot-cracking, strength, oxidation resistance, stress-rupture ductility, impact strength after long-time aging-corrosion resistance, and control of delta ferrite. These relationships have been used as a guide in determining the types and amounts of additions 'to make in a novel stainless steel which combines an excellent level of all the above properties, including especially an unexpectedly high resistance to hot-cracking.

Reviewing these findings in general terms, I have found a high resistance to .oxidathat, in the basic composition, tion and hot-cracking as well as good mechamcal properties are obtained when the chromium content is as high as it can be without giving rise to delta ferrite and too much sigma phase, and when nickel is as low as it can be without similarly producing delta ferrite and sigma.

Manganese should be maintained ata high level and in 'a particular relationship to the molybdenum content. Aluminum, copper and silicon should be maintained at a low level. The solid solution hardeners molybdenum, :tungsten and; vanadium could be added in relatively large amounts to obtain high rupture strength but must be limited by their effects on oxidation, rupture ductility and impact strength. Molybdenumand tungsten actually increase the resistance to cracking when used in the proper amounts, while vanadium does not adversely affect the cracking characteristics in any amount up to 5%." Of the ,austenite stabilizers and strengthening'elements, carbon .and nitrogen, nitrogen is to be preferred over carbon be cause nitrogen reduces hot-cracking. The carbide stabilizers titanium and columbium increase the susceptibility of the alloy to hot-cracking. Therefore, if a stabilizing addition is used to increase the strength or is required to prevent intergranular corrosion of as-welded structures, or materials heated in the sensitizing temperature range, the amount of titanium or columbium added should be restricted to as low a level as possible, consistent with the requirements; Because carbon is detrimental to hotcracking and also increases the amount of titanium or columbium needed for stabilization, there are two reasons for keeping the carbon level low. Certain elements, such as boron and zirconium, which are sometimes added to austenitic stainless steels to obtain particular characteristics, are especially potent in increasing hot-tearing and cracking, and, therefore, whenever these elements are added to an alloy in which the hot-cracking property is of special importance, care should be taken to restrict the amount of these elements to the lowest level consistent with attaining the desired result. The effect of hafnium on cracking is similar to that of zirconium.

In partial summary of the major points thus far presented, it has been found that Fe-Cr-Ni alloys with 1.5% Mo and 12% Mn have unusually high and unexpected resistance to hot-cracking. In the preferred range of 12% to 15% (Mn+Mo), molybdenumis limited on the high side by considerations of hot-cracking, oxidation resistance and impact strength to a maximum of about 3.75% and preferably 3.0%. On the low side, resistance to hot-cracking, rupture ductility and strength dictate a lower limit of 0.5% and preferably 1.0% M0. The broad range of manganese in these particular alloys is 75% to 15 and the preferred range is 9% to 14% Mn. If only small amounts of crack promoters are present, the manganese range may be somewhat lower, as'for example, 8% to 13%. The broad range of molybdenum is 0.5 to 3.75% in these alloys and the preferred range is 1 to 3%. Tungsten may be substituted for all or part of the molybdenum, but since it is more expensive and'provides no additional benefit, molybdenum is preferred. Vanadium is limited to 1% and preferably 0.5% because of oxidation. resistance. Columbiurn may be added in amounts of from 0.25% to 1.5% to stabilize the carbides or increase high temperature strength. Titanium also acts as a carbide stabilizer in-the range of 0.25% to 1.5%. Amounts of from about 0.25% to 0.5% or higher produce a marked increasein hot tensile ductility. Finally, in the rangeof 1.5% to 2.5 titanium the precipitation hardening phenomenon occurs, producing increased strength. Chromium must be at least 14% to provide oxidation and corrosion resistance but should not exceed 20% .because this would require too high a nickel content to maintain the composition balance. Almost all of the alloying elements in thesteel affect the balance of austeniteand ferrite. .F(c) asdefined should have a value of 7.5-11.5 if 2l0% delta ferrite is desired in the cast structure. For a fully austenitic structure F(c) should be greater than about 14.

The whole composition also influences the rate of sigma 'loys are given in Table VI. Not all of the alloys in these tables are within the scope of this invention as defined by 'the claims, for they may fail to meet one or more of the requirements set forth in the preceding discussion.

COMPOSITIONS OF ALLOYS IN WEIGHT PERCENT AND THEIR HOT-CRACKING SUSOEPTIBILITY RATING CPT Heat No. Test Cr Ni Mn Si M W V Ti Ch 0 .N Fe

Mold No.

7. 8 (16) (7) (11.5) (.15) (1. (.03) (.08) Bal. 8 5 16.3 14. 0 12. 0 (0. 15) 1. 58 0. 0062 0. 016 16. 4 14. 0 12.3 (0.15) 1. 48 (0.01) (0.015) 8.5 .16. 4 13. 8 12. 1 0.2 1. 57 0.050 0.015 8 7 16.5 13.9 11.9 0. 14 1. 51 0. 082 0. 041 9.0 16.3 14. 0 12.0 0.13 1. 53 0.070 0.026 8.5 17.0 16.6 15.0 0.10 0.006 0. 055 10. 2 (16) (14) 10. 7 (0. 15) 0. 039 0.073 9. 2 16. 7 15. 8 14. 2 0.22 0. 014 0. 021 9. 0 (16) (14) 10. 7 (0. 15) 0. 033 0. 043 8. 7 10. 4 15. 9 14. 3 0.13 0. 009 0.012 9. 4 (16) (25) 11.0 15) 0.006 0.016 9. 6 (16) (25) 11. 7 15) 0. 0035 0. 021 8. 5 (16) (20) 10. 6 15) 0. 0044 0. 019 9. 0 (16) (22) 8. 93 15) O. 078 0. 017 9. 4 (16) (22) 16. 0 15 0. 00.35 0. 024 9. 6 (16) (20) 20. 1 15) 0.0037 0.016 9.0 17. 1 16.0 10.5 0. 26 0. 032 0. 040

1 Mold No. for Cracking Inrlex=40. Parenthesis indicates nominal analysis. 2 3,000 pound melt.

Table VI COMPOSITION OF HEATS, WEIGHT PERCENT (BALANCE FE-i-IMPURITIES) Heat No. Cr N1 Mn Si O N Mo W V 13 Zr Ti Cb Ta 7431 16 20 10. 6 15 0044 019 2. 69 7454. 16.3 17.0 9. 43 21 039 041 2. 91 7458. 16 18 7. 80 15 031 015 1. 46 7459- 16 18 7. 85 15 040 018 1. 46 7460- 18 18 10. 3 15 040 023 3. 64 7462- 16. 0 17. 8 7. 76 16 .054 .098 1. 43 7467 15.2 16.9 9. 61 .32 048 11 2. 72 7470 16 20 9.02 15 .037 13 2.00 7471- 16 20 8. 92 15 .032 11 1. 39 7472- 16 20 8. 32 15 040 074 1. 36 7478- 16 20 9. 07 15 036 091 2. 08 05 7479- 16 20 8. 95 15 034 .12 2.03 (.05 7480. 16 20 9. 08 15 033 12 2. 03 05 7486- 16 20 8. 80 .15 042 .058 1.16 (.05 7487.. 16.6 20.7 11.5 .09 035 .15 2.14 (.05 7488.. See Heat N0. 7487 13 7489 16 20 8.62 15 044 12 2. 00 05) 7530- 16 20 8. 53 15 .035 .019 2. 12 05) 7531-- 16 20 9 54 .25 .030 .040 1. 91 7532-- 16 20 9. 32 .25 035 044 2.18 (.10) 7533-- 16 20 9. 91 25 .031 .053 (.10) 7534- 15.8 19.3 9. 67 29 .026 .052 2. 24 01 7535- 16 20 9. 45 25 028 045 2. 24 7571-- 15. 0 19.7 10.4 12 038 19 2. 25 7572- 16 20 10. 6 030 23 2. 23 7577.. 15 19. 5 8. 17 096 034 19 2. 40 VM545. 16. 7 12. 6 02 .017 15 2. 61 XMM2216 15.9 19. 7 10. 1 27 035 2. XMM2264 15.9 19. 7 9. 37 37 .031 056 2. 13 XML 12232 16. 0 20. 4 9. 03 37 031 10 2. 24 2239---. 16.0 19.5 8. 97 .31 .038 .071 2.16 2240. 15. 9 19. 4 9. 02 .31 034 094 2. 11 2265-.. 15.7 19.8 10.6 36 026 16 2. 22 2206 (wire) 15.8 19. 7 10. 7 .42 024 13 2. 17 101 15.7 20.0 10.3 48 025 13 2. 32 XMM2277 (wite) 15.9 20.2 11. 6 19 017 14 2. 32 2284 15.9 20.1 11. 0 20 021 .17 2. 27

With the information obtained from the series of 25 pound heats set forth in Table V it was possible to prepare and stu dy heats of commercial size having preferred compositions. One such heat is identified as XMM2054.

The tensile properties of heat XM-M2054 from room temperature to 2200 F. are listed in Table VII] The specimens were cut in the longitudinal direction from a 1%" thick hot-rolled plate which was solution heat treated one hour at 2000 F. There is a minor dip in ductility at about 1350 5. However, since the minimum reduction of area up to 2200 F. is greater than this Wrought alloy is suificiently ductile to Withstand all normal fabrication and service imposed strains, including annealing and stress-relief heat treatment of restrained joints in heavy Welded sections.

Table VII TENSILE PROPERTIES OF HEAT NO. XMM2054 1% THICK HOT-ROLLED PLATE SOLUTION TREATED 1 HOUR AT 2000 F. AND WATER QUENCHED (STRAIN RATE 750% PER HOUR, SPECIMEN 0.357 DIAMETER GAGE LENGTH 1 -RATE 750% PER HOUR; SPECIMEN 0.357 DIAMETER, 1% GAGE LENGTH) Test 0.5% Ultimate Percent Percent Heat No. Temp., Yield Strength, Elon- Reduc- F. Strength, p.s.i. gation tion of p.s.i. Area 7392 RT 26, 800 70, 600 v 62. l 78. 1, 100 13, 300 47, 200 V 40. 8 69. 3 2,009 4, 600 5, 600 67. 55.1 7403 RT 38, 600- 87,100 53. 3 69. 4 1, 100 20,800 67, 800 40. 4 50. 4 2, 000 6, 100 6, 700 46. 5 46. 3 7405 RT 33,100 79,200 64. 0 79. 5 1, 100 16, 000 57, 000 50. 0 69. 6 2, 000 5, 500 6, 300 51. 0 46. 2 7408 RT 29, 200 75,200 54. 8 78. 5 1, 100 14, 300 47, 700 39. 1 55. 4 2, 000 4, 300 5, 300 68. 9 48. 6 7410 RT 30,000 78, 500 50. 6 69. 1 1, 100 17, 300 59, 000 40. 2 58. 4 2, 000 5, 300 5, 800 62. 2 54. 6 7429 RT 25, 900 500 55. O 73. 9 1, 100 14, 500 46, 600 30.8 40. 9 2,000 5,000 5, 600 39.0 31. 6 7431 RT 29, 800 77, 600 54. 8 75. 7 1, 100 15, 500 54, 100 45. 3 6S. 8 2, 000 5, 600 6, 200 35. 0 34. 3 7432 RT 39, 500 92, 100 46. 0 66. 0 1, 100 23,050 63, 700 32. 9 46. 0 2, 000 6, 300 6, 300 39.0 36. 8 7440 RT 32, 600 82,000 51. 6 72. 4 r 1, 100 18, 600 59, 400 41. 7 63. 5 2,000 6, 200 6, 900 34. 5 29. 1 7441 RT 32, 800 81, 600 50. 2 71. 8 1, 100 18, 500 58, 190 36. 7 53. 9 2, 000 4, 500 5, 200 35. 9 43. 1

Heat XMM2054 was successfully worked by the conventional techniques of forging, rolling and wire drawing. 7

Table IX STRESS-RUPTURE TESTS OF XMM2054 11 THIGK'I-IOT- ROLLED PLATESOLUTION TREATED 1 HOUR AT 2000 F.

.AND WATER QUENOHED SPECIMEN 0.505 DIAMETER,

GAGE LENGTH 3 r 7 Minimum Rupture Rupture Reduc- Creep Test Stress, Time, Strain, tion of Rate, Per- Temp. F. p.s.i hours percent Area, cent per percent hour This type alloy has rupture strength and ductility comparable with the A151 type 316 steel, but it diifers from type 316 in being fully austenitic as cast and in weld deposits, the wrought base metal is more resistant to cracking in the heat-affected zone of welds, and the austenite does not transform to martensite even when deformed at cryogenic temperatures.

The 1200 F. stress-rupture and creep properties of certain of the weldable stainless steels of Tables V and VI are summarized in Table IV, which illustrates the efi ect of nitrogen on strength, and in Table X which shows the eiiects of various levels and combinations of Mo, W, V, Ti, Cb, C and N.

Table X SUMMARY OF 1200 F. STRESS-RUPTURE AND CREE? STRENGTHS'OF WELDABLE STAINLESS STEELS From these and other data it is possible to draw some conclusions about the eitects of dilferent alloying elements on the high temperature strength of these stainless steels. Simple Fe-Cr-Ni-Mn alloys, as illustrated by heats 7408 and 7429, are exceedingly weak with the alloy containing 20.1% manganese (heat 7429) having even lower strength than the alloy with 15.0% manganese (heat 7408). Substantial additions of 4.6% tungsten alone in heat 7440, 3.21% vanadium alone in heat 7441 and molybdenum from about 1.4% to 3.9% in several other alloys produced alloys of intermediate strength. Heats 7448, 7458, 7460, 7459 and 7411 containing increasing amounts of titanium from 0.14% to 1.15 show little or no increase in strength in this titanium range over alloys of similar molybdenum content without titanium. A significant effect of titanium on hot tensile. ductility will be discussed later. The three alloys in Table X which have the highest creep strength (7403, 7410 and 7451) are characterized by having one or both of the elements from the group columbium and Vanadium present in the base compoistion in combination with appreciable quantities of nitrogen and/ or carbon.

It has been indicated herein that when either'titanium or columbiurn, or both, is to be added to the alloy, a minimum of 0.25% be employed. Actually, smaller amounts of titanium and/or columbium could be present with some benefit, either as impurities or intentional additions, but their ehectiveness would not be as significant as in the preferred range. When colurnbium is employed in the alloys, it will be understood that normal commercial sources of colurnbium, such as ferro-ailoys thereof, may be used. Ordinarily, columbium so obtained will contain a small proportion of tantalum, and the final alloy may have tantalum present therein to this extent.

Many of the alloys in Table X were subjected to the RPI-type hot ductility test referred to earlier to determine their probable heat-affected zone behavior when subjected to the thermal cycles and stresses'associated with weld ing. Most of the alloys had a rapid recovery of ductility when tested, on cooling from the peak temperature, and would thus be expected to exhibit good weldability. Heat 7410 with the columbium addition and no molybdenum was marginal in this test. v 1

As examples of the electrodeand base metal welding characteristics of the basic Pe-CnNi-Mn-Mo alloys of this invention a number of laboratory and production 21 welds were made with covered electrodes in both wrought and cast base metal. Typical welding conditions are given in Table XI and the compositions of the deposits are listed in Table XII. In spite of the fact that all the LL WELD METAL SPECIMEN AS DEPOSITED. GAGE DIAMETER 0.505, LENGTH 3 Weld deposits were fully austen1t1c (1.e., no delta ferrite), 5 Rupture Rupture Reduction Minimum no fissures were observed at 30 times magnification on Stress, Time, hours IStraimt tll fArea, greep Rate, the polished surfaces of side-bend specimens after bendmen went iriiiii ing, either in the weld bead or the heat-affected zone of the base metal. Satisfactory MIG welds have also been 30, 000 5,2 13,8 30.7 0.10

H 25,000 1 8 21.0 34.2 0.0087 made using diameter bare wire with argon conta n- 20,000 832 5 5 2L0 0.00062 ing about 1% oxygen or pure argon lIl COIHUIICIDOH with 17,500 1, 903 1.0 11.8 0 21 a thin cathode stabilizing film on the wire.

Table XI TYPICAL CONDITIONS USED IN WELDING THE ALLOYS OF THIS INVENTION Travel Interpass Process Remarks Wire Current, Volts D.C. Atmosphere Speed, Tempera- Die... 1n. amps Polarity Inches] ture, F.

min.

Covered Eleetrodes Both titania and lime type 0031311155. A 100-120 24. 5 Reverse 200300 Some alloying elements frequently A2 145-160 22-27 d added to coating. X0 180 22 MIG Bare wire. Surface treated for cath- 34 2 0-3 0 24.28 ode stabilization. Me 280-320 24-28 TIG diameter tapered tungsten elec- 32 295-325 17 trode. z diameter filler Wire laid in groove. TIG %2 diameter tungsten electrode. None 80-90 11.5 do Heliu n1 4 Weld d in Butt weld between }4th1ck plates. one pass.

1 Argon plus about 1% oxygen.

Table XII COMPOSITIONS IN WEIGHT PERCENT OF FULLY AUS- TENITIC WELD DEPOSITS OF HIGHLY WELDABLE STAINLESS STEEL 1 Balance iron plus impurities.

The tensile properties of two of these welds are given in Table XIII at room temperature andelevated temperatures up to 2200 F, in-both the as-deposited and solution treated conditions.

Table XIII TENSILE PROPERTIES OF TITANIA DEPOSITS NO. 167.AND 170 XMN Although the primary use of my alloys is in high temperature applications, these steels possess characteristics which make them useful at subzero temperatures and, more specifically, for cryogenic applications involving the handling of liquefied gases. For example, because of the austenitic (face-centered-cubic) crystal structure, the alloys do not have a pronounced impact transition temperature as is common in ferritic steels. Moreover, the high nickel and manganese contents stabilize the austenite so that even after deformation at liquid nitrogen, hydrogen or helium temperatures the alloys do not transfrom to martensite, and thus they remain nonferromagnetic. This last characteristic was an important consideration in the selection of an alloy of this invention for a large hydrogen bubble chamber. The alloy chosen for this application has a nominal composition of 16% chromium, 20% nickel, 10.0% manganese, 2.25% molybdenum, 0.3%

TYPE COVERED ELECTRODE WELD 2054 CORE WIRE DIAMETER Test 0.2% Yield Ultimate Total Reduc- Condition Temp, Strength, Strength, Elongation of F. p.s.i. p.s.i. tion, Are

Percent Percent A W ld d RT 56,000 77, 600 42. 2 61.2 1, 100 31, 600 52, 300 30. 7 00. 8 1, 350 27,200 35, 600 53. 3 54. 7 1, 600 18, 500 19,900 50. 3 50. 8 1, 900 8, 900 9, 200 19. 0 24. 8 i 2, 200 4, 000 4, 200 27. 3 35. 0 Solution Treated 1 hr. at 2,000 RT 33,600 72,300 54. 6 72. 1 F. and water quenched. 1, 100 17, 600 50, 600 46.1 62. 9 1, 350 17,000 ,300 61. 9 68. 5 1, 600 16, 300 400 76. 9 69. 1 1, 900 8. 500 8, 900 38. 3 46. 9 2, 200 2, 900 4, 000 31. 8 33. 3

The soundness of the weld deposit was further demonstrated by the absence of microfissures on the surface of the room-temperature tensile specimen after test. It will be noted that there is a considerable decrease in strength up to 1350 F. with only a minor increase in ductility as a result of solution treatment, so that I usually prefer to use these welds in the as-deposited condition. The relatively good stress-rupture properties of typical Weld metal deposits of highly weldable alloys made in accordance with this invention are set forth in Table XIV.

silicon, 04% carbon and the balance essentially iron. This alloy has low thermal conductivity at cryogenic temperatures, is dimensionally stable and has reasonably good strength without requiring work hardening or heat treatment. The alloy therefore has properties which make it useful for vessels, pipes, valves and Dewar flasks for containing liquefied gases. The low temperature properties of an alloy of this invention having the nominal composition set forth above are presented in Table XV. This alloy had a nitrogen content of approximately 0.04%

23 due to pick up during melting; Additions of nitrogen of upto 0.35% may be made to increase the strength of this alloy composition as well as other alloy compositions of this invention.

Table XV low temperature austenite stability and oxidation resistance must usually be considered.

During the course of my investigations, in which the above effects were fully considered, I discovered that LOW TEMPERATURE PROPERTIES OF WELDABLE STAI NLESS STEELS Test Temperature 70 F. -160 F.

Cast Material Heat Treated, 1 Hr. 2,000

R.A., Percent".--

Charpy Keyhole Impact, Ft.'Lb Weld Deposit Heat Treated, None:

Tensile, p.s.i

R.A., Percent Charpy Keyhole Impact, Ft. Lb Wrought Material Heat Treated, 1 En,

Elong, Percent. R.A,, Percent Charpy Keyhole Impact, Ft. Lb

a All weld metal specimen, Transverse test, broke in weld. 0 Transverse test, broke in heat-affected zone of wrought base metal.

HIGH STRENGTH WELDABLE STAINLESS STEELS The alloys described above, particularly those of Tables 1V and X and similar alloys, have been found to be highly satisfactory for many purposes requiring reasonably strong, highly weldable stainless steels. There are, however, many applications requiring stainless steels having greater strength than the alloys described previously. For such applications, thehighly crack resistant alloy compositions described above are modified at some sacrifice in weldability to assure the required additional strength as described hereafter. In modifying these compositions the principles set forth previously are carefully followed so as to achieve optimum properties for the intended application.

Ordinarily, increased strength would be secured by increasing the proportions of the principal alloying strentheners, i.e., molybdenum, carbon and nitrogen, or by additions of tungsten, vanadium or columbium. The limits within which these constituents may be increased or added to the alloys are defined by the requirement that the high degree of weldability achieved by proper balance of the Fe-Cr-Ni-Mn-Mo base constituents,

in accordance with this invention, be retained as muchas possible. Thus, the optimum combination of manganese and molybdenum for rupture strength and resistance to hot-cracking in welding or casting has been described above and in FIG. 5, and to deviate significantly from this optimum ratio, will inevitably have a detrimental effect on these properties. In addition to the hot-oracle ing tendencies, the'eifects of alloying elements on one or more of the following factors, such as, delta ferrite formation, sigma precipitation, stress-rupture ductility,

"combinations of one or more of the elements from the "group boron, zirconium and vanadium taken in combination with nitrogen produced alloys of exceptionally high strength while still retaining excellent weldability,

r" as will be evident from the following examples and description.

The effects of boron, zirconium and vandium in addition to nitrogen were investigated singly and in various combinations with the results given in Table XVI. From results onlheat 7441 (Table X) with 3.2% V, it would have been anticipated that 0.57% V would have had a negligible effect in heat 7471, but unexpectedly, the

vanadium and nitrogen interacted and produced a strength willbeyond the sum of the eifects of the individual elements. Similarly, boron plus nitrogen (heats 7467, 747.8 and zicroniumplus nitrogen (heat 7480) had individual strengthening effects, but the combination of boron, zirconium and nitrogen produced onexpectedly high strength and excellent ductility in the alloy of heat 7479. It is. apparent that steels having excellent properties which; exceed those of commercially.

produced stainless steels can be obtained by employing one or more of the elements from the group consisting of boron, zirconium and vanadium taken in conjunction with high nitrogen in thecrack resistant alloy base. From these and other tests the useful ranges of these elements have been determined to 'be 0.002% to 0.03% boron, 0.002% to 0.06% zirconium, 0.15% to 1% vanadium, and 0.10% to 0.35% nitrogen. It is the unique combination of manganese and molybdenum, in the alloys of this invention, that enables me to obtain a high degree of weldability in these high strength alloys, in spite of the presence of boron and zirconium, which in FIG. 3 are shown to be detrimental in the simple iron-chromiumnickel alloys insofar as hot-cracking is concerned.

Table XVI SUMMARY OF 1200 F. STRESS-RUPTURE AND CREE]? PROPERTIES OF WELDABLE STAINLESS STEELS WITH ADDITIONS OF BORON, ZIR- OONIUM, VANADIUM AND NITROGEN 1 Residual boron from impurities in the charge or crucibleNo intentional addition.

( )=aim composition.

The preferred alloy composition adds to the previously defined Fe-Or-Ni-Mo-Mn alloy base the beneficial strengthening effects of all of the elements of the group boron, zirconium and vanadium in conjunction with nitrogen. The marked improvements in rupture and creep strength produced by the combined addition of B, Zr and V at a given N level are shown in FIGS. and 11. Curve A in each case was obtained in testing an Fe-Cr-Ni-Mo-Mn alloy base containing nitrogen. Curve B was obtained in testing the alloy base with additions of B, Zr," V and N. Not only are the curves raised by the combination of these addition elements, but the increased slope in the initial portion of each curve B indicates a synergistic interaction between (B, Zr, V) and N. These elements within the preferred composition range, by a mechanism not fully understood, produce a steel of unusually high strength and ductility. This unique alloy containing boron, zirconium, vanadium and nitrogen is an extremely desirable highstrength stainless steel which is more weldable than some of the commercially available stainless steels having even lower strength.

A high strength stainless steel alloy in which the alloy constituents are balanced to provide the desired high level of strength without forming undue amounts of sigma with its consequent embrittling effect, comprises, from'l4% to 16% chromium, from 20% to 24% nickel, from 8% to 13% manganese, from 1.5% to 3% molybdenum, the total of manganese and molybdenum being from 11% to 16%, from 0.15% to 0.35% vanadium, from 0.003% to 0.02% boron, from 0.003% to 0.04% zirconium, from 0.02% to 0. 06% carbon, from 0.2% to 0.25% nitrogen, up to .4% silicon, and the balance essentially iron and small amounts of incidental impurities.

Another preferred range of composition of the high strength weldable stainless steel alloy is -17% Cr, 18.5-22% Ni, about 813% Mn, 15-30% M0, the total of manganese and molybdenum being from 11% to "1 6%, 0.l50.35% V, 0.004-0.02% B, 0.005 to 0.04% Zr, 0.10 to 0.25% N, 0.01 to 0.06% C, up to 0.4% Si and balance iron and incidental impurities. A particularly useful high strength alloy for steam turbine piping applications contains about 16% Cr, about 20% Ni, about 11% Mn, about 2.25% Mo, about 0.25% V, about 0.006% B, about 0.015% Zr, about 0.04% C, about 0.20% N, less than about 0.25 Si, and the balance iron and small amounts of incidental impurities.

Even higher strengths can be provided by employing quantities of boron and Zirconium exceeding the preferred ranges above specified, but for applications involving difiicult Welding or casting operations, boron and zirconium shouldbe kept at the specified low levels to avoid hotcracking of weld deposits, heat-aifected zone cracking of base metal during welding and hot-tearing of castings as previously explained.

The powerful effect of nitrogen on rupture and creep strength of steels containing B, Zr and V is shown by curve B in FIGS. 10 and 11, as mentioned previously, and the data in Tables XVI and XVII. Rupture strength approaches a maximum at about 0.2% nitrogen. The corresponding 1000'hour stress-rupture elongation at 1200 F. drops rapidly from about to 40% between 0.02% and'0.04% N, as illustrated'in FIG. 12, but decreases very slowly at higher nitrogen contents up .to 0.13 and higher. Thus, at the high strength level corresponding to 0.19% to 0.23% nitrogen, the elongation is greater than 30%. In general, stress rupture elongation is a minimum in ,tests lasting about 1-00 to 300 hours. In tests lasting about 1000 hours and longer, the ductility increases again, contrary to general experience in stainless steel, as illustrated by the typical stress-rupture tests results set forth in Table XVII.

' Table XVII STRESS-RUPTURE PROPERTIES OF HIGH STRENGTH WELDABLE STAINLESS STEEL HEAT 7489 FORGED DIAMETER BAR SOLUTION TREATED 1 HOUR AT 1900 F.

Reduc- Minimum Test Stress, Rupture Rupture tion of Creep Te1np., p.s.i. Time, Strain, Area, Rate,

F. hours percent percent percent per hour 1 Test on heat 7488.

The 1200 F. stress-rupture and creep properties of heat 7489 in the above table may be compared with the similar properties of other high strength weldable stainless steels of this invention and with those of the highly weldable, stainless steels of this invention in Tables IV, X, XVI and XVIII where the 1000 hour properties are summarized.

a? Table XVIII SUMMARY OF 1200 F. STRESS-RUPTURE AND CREEP STRENGTHS OF HIGH S'ltqEglfii'gH WELDABLE STAINLESS It is a further characteristic of these high strength stainless steels that stresses higher than 62.5% of the yield strength and higher than 25% of the tensile strength can be applied without exceeding the boiler code stresses for minimum creep rate or stress-rupture life. For a fuller utilization of the high temperature capabilities of these alloys, it is, therefore, desirable to find ways of increasing the short-time yield and tensile strengths. For service temperatures up to about 1200 F., I have found that a fine austenite' grain size produced by adequate prior hot or cold working and a low temperature solution heat treatment in the range of about 16 50 to 1800 F. will increase the yield strength of Wrought material to equal or even exceed that of argon shielded weld deposits or of the Weld joint in tungsten inert gas Welded plates.

Adequate prior hot Working in these fully austenitic materials is achieved by control of initial soaking temperature, and holding time, amount of the reduction, tfinishing temperature, and reheating schedule. Preferably, the initial working temperature should only be high enough to provide ease of deformation and the finishing temperature should be low enough so that some hotcold Working occurs. Under these conditions the steel can be fully recrystallized to a uniform grains-ize finer than ASTM N0. 6 by solution treatments for one hour at temperatures in the range of about 1650 F. to 1900 F. When the steel is solution treated below about 1775 F., particles of an undissolved precipitate contribute to the yield and tensile strength. Tensile properties of hotrolled and solution treated plate made from the high strength stainless steel of this invention are set forth in Table XIX.

Table XIX probably the sigma phase. Upon subsequent solution treatment tor one hour at temperatures between about 1 650 F. .and 1-800" F., the matrix recrystallizes and a part of the precipitate remains as small spheroidized particles dispersed throughout the grains. These particles provide barriers to slip and account for the higher yield strengths at 1025 F. of the samples aged at 1350 F. and 1500 F. prior to the 1700 F. solution treatment (see Table XIX). However, this precipitation prior to testing has little effect on the long time creep or rupture properties because aging occurs at elevated temperatures in service in any event. I r

The tensile properties of a high strength austenitic stainless steel of this invention, containing boron, zirconimm, vanadiumrand a high nitrogen content of 0.23% were determined over a wide range of temperatures extending as low as 320 F. and as high as 2000 F. (see Table XX). The strength and ductility of this alloy at cryogenic temperatures (-320 F.) was quite remarkable. Since the boron and zirconium make little or no contribution tostrength or ductility at such low temperatures, alloys containing only nitrogen, in addition to the basic iron-chromium-nickel-manganese-molybdenum a'lloy (with or without vanadium), Will display essentially the same good low temperature tensile proper-tics as the alloy of Table XX. The object of using an alloy ithout the boron and zirconium addition for cryogenic applica tions would be to secure better weldability.

Table XX TENSILE PROPERTIES OF HIGH STRENGTH STAINLESS STEEL HEAT 7572% DIAMETER FORGED BARS SOLU- TION TREATED 1 HOUR AT 1700 F.

Test 0.2% Yield Ultimate Total Reduction Temp. I Strength, Strength, Elongation, of Area, p.s.1. p.s.i. Percent Percent 320 135, 500 200, 000 57. 0 53. 5 RT 84, 490 116, 200 as. 1 55. 0 1, 025' 53, 600 82, 000 20. 5 57. 3 1, 350 48, 800 61, 100 24. 8 39. 2 1, 600 35, 300 37, 500 36. 9 49. (a 2, 000 9, 500 700 26. 8 30. 3

In structural and pressure vessel design it is usually preferred that the yield strength of the weld metal be greater than that of the base metal to avoid strain concentration in the event of overstressing. I therefore prefer to select a solution temperature for the base material :prior to Welding high enough to give a yield strength slightly lower than that of the type weld deposit to be used. I prefer not to solution treat the welded joints beceause this results in a substantial reduction in their yield and tensile strength. The high strength of as deposited Weld metal is obtained in spite of a coarse grain size and may be due to such eifects as the cellular structure with- TENSILE PROPERTIES OF COMlliERCIALLY HOT-ROLLED $4" PLATE OF HIGH STRENGTH WELDABLE STEEL HEAT XMM2265 Heat Treatment Test 0.2% Yield Ultimate Elongation Reduction Temp., F. Strength, Strength, in 2", of Area,

Hours Terrlr p Hours T erlr l p. p.s.i. p.s.i. percent percent 1 1, 850 RT 44, 000 102, 000 43 48 1 1, 800 RT 45, 000 102, 000 43 5t) 1 1, 750 RT 48, 000 104, 000 42 51 1 1, 700 RT 50, 000 105, 000 37 48 20 1, 350 1, 025 37, 000 82, 000 32 47 p 20 1, 500 1, 025 35, 000 82, 000 34 44 1 1, 700 1, 025 33, 000 82, 000 51 1 1, 850 1, 025 31,000 81,000 35 48 1 1, 900 1, 025 27,000 78, 000 44 50 For additional strengthening or in articles such as large forgings where it is diflicult to introduce unif-onm work hardening, a heat treatment for about 20 hours in the range of about 1300 F. to 1550 F. will precipitate substantial amounts of a second phase as dispersed particles,

in the grains, segregation during freezing or lattice imperfections resulting from the solidification rocess. A solution treatment seems to markedly reduce such strengthenmg mechanisms. In the absence of substantial amounts of hot or cold Working, which are impractical to apply 

1. AN AUSTENITIC STAINLESS STEEL ALLOY OF GOOD WELDABILITY COMPRISING, BY WEIGHT, FROM 14% TO 20% CHROMIUM, FROM 15% TO 30% NICKEL, FROM 7.5% TO 15% MANGANESE, FROM 0.5% TO 3.75% MOLYBDENUM, THE TOTAL OF MANGANESE AND MOLYBDENUM LYING IN THE RANGE FROM 9% TO 16%, FROM 0.01% TO 0.08% CARBON, FROM 0.01% TO 0.35% NITROGEN, UP TO ABOUT 1% SILICON, UP TO 1% VANADIUM, UP TO 0.03% BORON, UP TO 0.06% ZIRCONIUM, UP TO 1.5% COLUMBIUM, UP TO 2.5% TITANIUM AND THE BALANCE IRON AND SMALL AMOUNTS OF INCIDENTAL IMPURITIES, THE $(C) VALUE OF THE ALLOY BEING GREATER THAN 1 TO INSURE AN ADEQUATE LEVEL OF STRENGTH AND RUPTURE DUCTILITY AND BEING LESS THAN 3.75 TO INSURE THAT SERIOUS LOSS OF IMPACT STRENGTH AFTER LONG TIME SERVICE AT ELEVATED TEMPERATURES DOES NOT OCCUR, THE F (C) VALUE OF THE ALLOY BEING GREATER THAN 14 TO ASSURE THAT THE ALLOY IS FULLY AUSTENITIC. 